1. Field of the Invention
Embodiments of the present invention relates to antenna port pairing and subcarrier mapping, and more particularly to a method and a system for antenna port pairing and subcarrier mapping for downlink transmit diversity, which provide better protection for important modulation symbols and improve the reliability of transmission.
2. Description of the Prior Art
Transmit Diversity (T×D) is a very important multi-input and multi-output (MIMO) transmission mode, especially for user equipments (UE) with high mobility and low Signal to Interference plus Noise Ratio (SINR).
Generally, a communication system needs to adopt channel coding to check and correct errors so as to improve the reliability of the data transmission. For output of the channel encoder, different bits have different levels of importance. For example, the systematic bit generally has higher importance while the parity bit generally has lower importance.
The downlink transmit diversity in 3GPP Long Term Evolution (LTE) adopts a scheme combining the Space-Frequency Block Code (SFBC) and Frequency Switch Transmit Diversity (FSTD), whose coding matrix is shown in FIG. 1.
As shown in FIG. 1, a fixed antenna port pairing is adopted in this scheme, in which antenna port 0 and antenna port 2 are paired to transmit one SFBC block, and antenna port 1 and antenna port 3 are paired to transmit one SFBC block.
The reason why LTE adopts fixed antenna port pairing is that the densities of the Reference Signals (RS) corresponding to the antenna ports are different.
FIGS. 2a, 2b, 2c and 2d are schematic diagrams showing distributions of RSs of antenna ports 0, 1, 2, 3 respectively.
The reason why LTE adopts this kind of antenna port pairing mode (one high density RS antenna port together with a low density RS antenna port) is to achieve the performance of balance between two SFBC blocks, i.e. to protect the two SFBC blocks in a same way.
The structure of the LTE transmit diversity transmitter is shown in FIG. 3. The data symbols output by the modulator include two parts: systematic symbol S; and parity symbol P.
The systematic symbol is more important than the parity symbol.
However, since frequency domain distances from different subcarriers to subcarriers in which RSs are located are different, different subcarriers have different reliabilities. Generally, the smaller the frequency domain distance from the subcarrier to the subcarrier in which the RS are located, the higher the reliability of the subcarrier is.
However, according to the LTE transmit scheme different reliabilities between subcarriers are not taken into account and the importance of data symbols is not differentiated when the antenna port pairing and subcarrier mapping is performed. Therefore, important data symbols may be mapped to low reliable subcarriers and thus optimal performance cannot be obtained.
For the single antenna system, there is an improved subcarrier mapping scheme to solve the above problems, which maps comparatively important data symbols to subcarriers which have comparatively small frequency domain distances with the subcarrier in which the RS is located. However, this method is not suitable for multi-antenna transmit diversity scheme. Examples are given below.
FIG. 4 is a schematic diagram showing a distribution after the symbols in the transmit diversity coding matrix are mapped to the corresponding antenna ports and subcarriers according to the fixed antenna port pairing in a 4-antenna system. As shown in FIG. 4, R0, R1, R2 and R3 are reference signals on antenna ports 0, 1, 2 and 3 respectively. Since antenna ports 0 and 2 are a fixed pair and antenna ports 1 and 3 are a fixed pair, taking into account of the arrangement of the RSs at the antenna ports in slot 1, in slot 1, after the symbols are mapped to antenna ports and subcarriers, as shown in FIG. 4, if the systematic symbol S0 is mapped to the position nearest to the RS R0 at the antenna port 0, the systematic symbol S0* cannot be mapped to the position nearest to the RS R2 at the antenna port 2 because of the characteristic of the space frequency code. On the contrary, the parity symbol −P0* is mapped to the position nearest to the RS R2, i.e., —P0* is better protected than S0*, which will lead to the loss of the system performance. Similarly, −P1*/−P2*/−P3* is better protected than S1*/S2*/S3*.
Of course, the above is only a kind of subcarrier mapping mode. However, as long as the antenna ports are paired fixedly, there will be the above problem and in other multi-antenna systems, there are similar problems.